Process for treatment for high speed photographic materials

ABSTRACT

A process for the treatment of a photographic material particularly a high speed photographic material, comprising an emulsion containing silver halide grains the treatment being to reduce the sensitivity of the material to ionizing radiation comprises prior to exposure, subjecting the material to a treatment that reduces the surface image dispersity so that fewer centers develop on each grain. The process may involve subjecting the photographic emulsion to physical pressure to deform the grains to introduce internal trapping centers for latent image formation so that a proportion of the latent image can be formed within the grains rather than on the grain surface.

FIELD OF THE INVENTION

This invention relates to photographic materials and to a process forreducing their sensitivity to ionising radiation.

BACKGROUND OF THE INVENTION

It is well known that photographic materials, particularly high speedphotographic emulsions, are sensitive to ionising radiation. Sinceionising radiation is present throughout the environment, contact withthe photographic material is unavoidable.

PROBLEM TO BE SOLVED BY THE INVENTION

U.S. Pat. No. 4,264,724 describes a process for increasing thesensitivity upon exposure to light of a photographic emulsion byexposing the silver halide emulsion during preparation to an ionizingradiation. The effect is said to produce a silver halide gelatinemulsion having grains of emulsion containing in their interior sublatent image nuclei for providing sensitivity on exposure to light.However one of the effects of ionizing radiation on a photographicmaterial between its preparation and use is premature ageing throughhigh fog. In addition there is a significant increase in granularitythat is particularly severe for color negative materials.

U.S. Pat. No. 5,096,804 discloses that silver halide emulsionssensitized by the addition of a gold sensitizing agent and a sulphursensitizing agent are subject to fog and deterioration in graininess dueto natural radiation and discloses that film having a lower totalgold/silver weight ratio exhibited less fog and deterioration ingraininess than films in which the ratio is higher.

U.S. Pat. No. 5,284,740 discloses that the increase in fog and decreasein sensitivity due to radiation of a silver halide color photographiclight sensitive material can be mitigated by including a sulphurcontaining compound bearing an organic heterocyclic group.

The inventors of the present invention have found that the highgranularity is caused by the dispersity of latent image over the grainsurface, which results in several independently developable centres sothat the grain produces an abnormally large dye cloud that is bad forgranularity. This high granularity occurs at low densities near fogwhere it is particularly noticeable in the final image.

The present invention provides a solution to the problems of graininessand fog due to radiation by the provision of a process for treatingphotographic materials to reduce their sensitivity to ionisingradiation.

SUMMARY OF THE INVENTION

According to the present invention there is provided a process for thetreatment of a photographic material, particularly a high speedphotographic emulsion, which process comprises prior to exposure,subjecting the material, to a treatment that reduces the surface imagedispersity so that fewer centres develop on each grain.

ADVANTAGEOUS EFFECT OF THE INVENTION

Photographic emulsions treated in accordance with the invention havereduced granularity when subjected to ionising radiation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are graphs showing the variation in granularity as afunction of exposure to ionising radiation for two high speed emulsions.

DETAILED DESCRIPTION OF THE INVENTION

The terms granularity and graininess are well known in the photographicart and granularity may be measured by an instrument known as a microdensitometer.

The term internal latent image is also well known in the photographicart.

In one form of the invention the photographic emulsion is subjected tophysical pressure to deform the grains to introduce internal trappingcentres for latent image formation so that a proportion of the latentimage can be formed within the grains rather than on the grain surface.The internal traps only function as latent image sites under conditionsof high intensity encountered with an ionising radiation.

The process may be effected by introducing internal sites by physicallydeforming the grains or alternatively by introducing mild latticedeformities during precipitation and grain growth.

To effect the appropriate amount of deformation of the silver halidegrains the pressure may be from 3×10⁹ to 18×10⁹, preferably from 6×10⁹to 12×10⁹.N/m².

The period of time for which the pressure is applied is not critical andmay conveniently be from one second to 5 minutes, preferably from 10seconds to one minute.

The photographic material to be treated by the process of the inventionmay be a silver halide color negative material, particularly one havinga speed above 400 ASA or above 1000 ASA.

Typically, the material may comprise at least one red sensitive silverhalide emulsion layer, at least one green sensitive silver halideemulsion layer and at least one blue sensitive silver halide emulsionlayer on a support.

At least one of the layers may contain silver iodide, for example in anamount of not less than 12 mol %. The photographic materials may be anyof those described in U.S. Pat. Nos. 5,284,740; 5,096,804 and 4,692,401which are included by reference.

The invention is illustrated by the following Examples.

EXAMPLE 1

High speed emulsions used were Kodak (Registered Trade Mark) PFM 3528and PFM 3516 which are high speed green sensitive emulsions of speeds1000 and 800 respectively.

PFM 3528 is a generic 1.5% run iodide, 2% dump iodide polydisperse fastmagenta AgBrI tabular grain emulsion of thickness 0.15 micrometers.

PFM 3516 is a CAKI emulsion 3.60% iodide of thickness 0.14 micrometers.

The film strips were subjected to pressure before exposure using a MarkII Abrasion Sensitometer.

The film strip was clamped to the circumference of a 10 cm diameterwheel with the emulsion side in contact with the rubberised surface. Thewheel rotated whilst pressure was applied over the whole length of the12 inch film strip by a stylus (diameter of tip approximately 10micrometers). The stylus was displaced by approximately 9 micrometersafter each revolution so that, in essence, over a 2 minute period araster pattern was drawn on the film strip giving a pressured area ofabout 9 mm wide. The stylus applied pressure through the back of thecoating in order to smooth out the pressure pattern across the width ofthe film strip so that the raster pattern would not interfere withmeasurements of granularity.

Preliminary tests indicated that pressures of 50 g gave no noticeablechange in sensitometric curve shape and pressures of 150 g gave a smalldecrease in contrast and Dmax and a small increase in fog density.

The films were either exposed to light via a step wedge for 0.01s on theDF sensitometer or exposed uniformly to cobalt (60) radiation at theNational Radiological Protection Board to give a control strip with zeroexposure and several strips with radiation exposures up to Emax=0.8R.The films were processed in the Kodak T 174 (Registered Trade Mark)process to give a dye image and all the film samples relating to aparticular coating were processed simultaneously for both types ofexposure to minimise processing variability. Data shown for pressureareas were always taken from the same film strip to avoid processing andsilver laydown uncertainties.

The dye densities were measured on the SPADE densitometer and thegranularities were measured as a fluctuation in the dye density (iesigma D) using the LEITZ granularity instrument. The latter used a 48micron diameter circular aperture and 2000 data points and in thisReport the granularity values relate to 1000*sigmaD.

The data for a single measurement were obtained from an areaapproximately 1 mm wide across the width of the film strip and 5 mm longand measurements were made every 110 mm along the length of the filmstrip, which corresponded with the steps of the wedge. In the case ofthe cobalt (60) exposures, where a wedge was not used, the sameprocedure was adopted and the multiple measurements offered animprovement in confidence. A investigation of the effect that thepressure had on the granularity was carried out by making severalmeasurements across the film strip at distances of 1 to 2 mm apart forevery exposure step, for every unpressured area and for every pressuredarea. The SPADE and the LEITZ instruments used a blue coloured filter tosuit the yellow coupler dye and all densities were corrected by removinga base density of 0.02.

Referring to FIGS. 1 and 2: these are graphs showing the variation ofgranularity for the two high speed emulsions PFM 3516 and PFM 3528.

The granularity is shown as a function of the exposure go to ionisingradiation for areas of film that were either given no pressure or givena load of 100 g. The average values and the standard deviations areshown.

The results show that the emulsion which had been subjected to thepressure treatment had a lower granularity on exposure to cobalt (60)that the one that had not been treated.

What is claimed is:
 1. A process for reducing the sensitivity toionizing radiation of a photographic material comprising an emulsioncontaining silver halide grains, which process comprises, prior toexposure, subjecting the material to a treatment that reduces thesurface dispersity so that fewer centres develop on each grain, thetreatment comprising subjecting the photographic emulsion to theapplication of physical pressure from 3×10⁹ to 18×10⁹N/m² to effectdeformation of the grains such that internal trapping centres for latentimage formation are introduced so that a proportion of the latent imagecan be formed within the grains.
 2. A process as claimed in claim 1wherein the photographic material is a high speed photographic emulsionhaving a speed of at least 400 ASA.
 3. A process as claimed in claim 1wherein the photographic material is a silver halide color negativematerial having a speed of at least 1000 ASA.
 4. A process as claimed inclaim 1 wherein the pressure applied to the photographic emulsion toeffect the deformation of the grains is from 6×10⁹ to 12×10⁹N/m².